My overarching goal is to understand the molecular mechanisms underpinning collective behaviors. To explore this, I will study quorum sensing, the process of cell-cell communication in the model bacterium Vibrio harveyi: Using quorum sensing, bacteria produce, detect, and respond to extracellular signal molecules (autoinducers) to coordinate gene expression on a population-wide scale. In V. harveyi, five regulatory small RNAs (called Qrr sRNAs) are produced at low cell densities and act to destabilize the luxR mRNA encoding the master quorum-sensing regulator, LuxR. My goal is to understand what features having five Qrr sRNAs rather than one sRNA provides the V. harveyi quorum-sensing circuit, and, more generally, what features sRNAs rather than regulatory proteins provide to signaling networks. Specifically, I will address the following points. 1) Define the transcription factors that individually control each qrr sRNA gene. The five c/rr sRNA gene promoters are divergent, and each gfrrsRNA gene shows a distinct pattern of gene expression, suggesting that different transcription factors regulate expression of each Qrr sRNA gene. I propose to identify these transcription factors and characterize their roles in the V. harveyi quorum-sensing response. 2) Identify mRNA targets that are exclusive to each Qrr sRNA. I will identify genes regulated exclusively by each of the V. harveyi Qrr sRNAs and determine the quorum-sensing functions of the identified targets. 3) Determine how different Qrr sRNA affinities for target mRNAs establish an ordered program of quorum-sensing gene expression. I propose that the Qrr sRNAs bind with different affinities to mRNA targets and this affects the timing of the various quorum-sensing response outputs. I will compare binding strengths of the Qrr sRNAs with known V. harveyi mRNA targets, define the sequence requirements for Qrr sRNA binding, and how altering those binding properties affects the timing of quorum- sensing gene expression. An important practical aspect of our quorum-sensing investigations is that they are leading to the development of anti-quorum-sensing therapies for use as alternatives to traditional antibiotics. Similarly, a molecular understanding of quorum sensing in beneficial bacteria, such as those that produce medical, agricultural, or industrial products of importance, could facilitate the development and implementation of pro- quorum-sensing strategies that improve industrial-scale production of commercially relevant natural products.